Abstract: Research into the atomic structures of metal materials in the liquid state, their dynamic evolution versus temperature until the onset of crystal nucleation has been a central research topic in condensed matter physics and materials science for well over a century. However, research and basic understanding of the atomic structures of liquid metals are far less than those in the solid state of the same compositions. This review serves as a condensed collection of the most important research literature published so far in this field, providing a critical and focused review of the historical research development and progress in this field since the 1920s. In particular, the development of powerful synchrotron X-ray sources and the associated experimental techniques and sample environments for studying in-situ the atomic structures of different metallic systems. The key findings made in numerous pure metals and metallic alloy systems are critically reviewed and discussed with the focus on the results and new understandings of structural heterogeneities found inside a bulk liquid, at the liquid surface or liquid-solid interface. The possible future directions of research and development on the most advanced experimental and modeling techniques are envisaged and briefly discussed as well.

Key words: Synchrotron X-rays, Atomic structures, Metal alloys, Liquid state, Molecular Dynamics modelling